Medical devices often include a therapy generator and one or more elongated leads, coupled thereto, which are positioned within a patient's body to deliver therapy from the generator. Such therapy may be in the form of electrical stimulation, delivered via electrical conductors extending through a lead body, or fluid infusion, delivered via a lumen extending through a lead body. Some examples of electrical stimulation include pacing and defibrillation; some examples of fluids, which may be infused, include drugs, nutrients, and genetic materials. In many applications, leads are inserted into a cavity or space within a body, for example a blood vessel or an interstitial channel or pocket, formed subcutaneously or submuscularly, where the lead must be retained for a period of time in order to deliver the therapy. Therefore it is desirable to provide lead retention means allowing insertion or forward motion of lead, to position the lead within the space, while preventing retraction or rearward motion of the lead during therapy delivery.